1. Field of the Invention
The present invention relates to a method of making a microstructure in a glass or plastic substrate according to hot-forming or hot-shaping technology, in which the microstructure is formed in viscous glass or plastic by pressing a forming tool, whose surface is structured according to a negative of the microstructure to be made, into the glass or plastic substrate and then removing the forming tool from it after completing the structuring. The invention also relates to the forming tool used to make this sort of microstructure according to hot-forming technology.
2. Related Art
Glass or plastic substrates, whose surfaces have a microstructure, are required in certain new engineering fields. For example, plate glass for precision applications provided with a microstructure of even higher precision is employed, especially in display devices, illuminating systems and in optical and medical engineering applications and in sensor technology. Display panels for newer flat screen technology are of special significance (PDP=plasma display panels or PALC=plasma addressed liquid crystal).
Microchannel structures in the form of several parallel channels are produced in these so-called channel plates.
This sort of channel plate is illustrated in the detailed view shown in FIG. 6. The channel-shaped microstructure appearing in this figure is economical and is used in large-scale manufacture of different sized displays (display screen diagonals up to 55xe2x80x3). The structure dimensions are in the following ranges independently of the screen format: pitch Xxe2x80x3=150 to 650 xcexcm; height Y=100 to 250 xcexcm; and width Z=20 to 50 xcexcm. For example, about 5760 channels with a spacing of xe2x80x9cXxe2x80x9d, the so-called pitch, of about 161 xcexcm, a height xe2x80x9cYxe2x80x9d of 150 xcexcm and a width xe2x80x9cZxe2x80x9d of 30 xcexcm with tolerances of a few xcexcm over a length of about 520 mm are made for a 42xe2x80x3 HiVision PDP display.
The problems with other engineering applications are similar.
Different methods for forming a microstructure are known. In one method the walls are applied to the substrate in several layers one after the other in a screen-printing process. This process requires a considerable effort and is expensive.
In another method the substrate surface is structured by a sand blasting or grinding method. In the grinding method a high-precision multi-disk grinding module is employed, which has a plurality of high precision grinding disks mounted on a common spindle spaced axially from each other by spacing rings.
Forming a microstructure in plasticized glass or plastic by a so-called hot forming method by means of a forming tool with a suitably structured surface is also known. This can be performed both in the glass material directly from the melt or plastic, the so-called prototype hot forming process, and in the conversion hot forming process with a solid substrate, as is described in DE 197 13 312 A1, in which a local heating of the structuring surface of the forming tool for melting the substrate surface is performed immediately prior to the shaping process.
The surface of the forming tool must be shaped according to the negative of the predetermined structure to be formed in the substrate in order to be able to form that structure in the substrate. The negative surface structure corresponding to the channel structure of FIG. 6 is illustrated in FIG. 5 for the described embodiment of a PALC channel plate.
During the forming process the low viscosity glass (or plastic) adapts itself to the structure of the forming tool. The selection of the forming tool used for this purpose is a matter of compromise. The wetting of the melted liquid phase on the forming tool must be great enough so that the glass or the plastic is as completely forced into the structured surface on the forming tool in operation as possible. On the other hand, the glass or the plastic to be structured must be able to separate again from the forming tool material after formation of the desired xcexcm-scale structure i.e. the wetting should thus be poor. In practice materials that have very good wetting behavior produce good xcexcm-scale structures, but the adhesion experienced during separation causes breakage of the formed structures.
The required xcexcm-geometry of the formed structures is poor when the wetting of the materials is poor. On the other hand, the probability of breaking the formed xcexcm-structures on separation is minimal.
Up to now it has not been possible to find a combination of materials, which satisfy the above-described requirements in an optimum manner, i.e. that is which guarantee that the negatively structured surface on the forming tool is completely filled when the forming tool is pressed into the substrate and that the structure produced is not broken when the forming tool is withdrawn from the structure, i.e. on shaping of the structure.
It is an object of the present invention to provide a method of making a microstructure in a glass or plastic substrate of the above-described type, in which the negatively structured surface of the forming tool fills with melted glass or plastic as completely as possible during formation of the microstructure with the forming tool, and in which the formed structure is not damaged when the forming tool is removed from the substrate.
This object and others, which will be made more apparent hereinafter, are attained in a method of making a microstructure in a glass or plastic surface by hot-forming technology comprising structuring the forming tool surface according to the negative of the microstructure to be produced; pressing the forming tool surface structured during the structuring into a viscous glass or a viscous plastic substrate and then, after the structuring, removing the forming tool from the structured glass or plastic surface.
The forming tool used in the method according to the invention is provided with an at least partially porous base material with an open pore structure, which acts on the structure during formation of it with an under-pressure and during removal of the forming tool from it with an overpressure.
The forming tool according to the invention for making a microstructure in a glass or plastic surface by hot-forming technology comprises a base body, an operative layer applied to a surface on one side of the base body and means for supplying or withdrawing air from another surface on an opposite side of the base body. The base body comprises a porous base material with an open pore structure and the operative layer comprises a gas-impermeable material structured according to a negative of the microstructure to be produced by the forming tool in order to form depressions or grooves that extend through the operative layer to the base material with the open pore structure.
Thus an under-pressure or overpressure is produced in the depressions or grooves of the negative structure of the forming tool in the method according to the invention, since air is drawn through or forced through the bottom of the depressions or grooves.
The sucking or drawing of air, the low pressure or under-pressure in the grooves or depressions, helps the process of filling the grooves or depressions with glass or plastic material. In contrast the blowing or forcing of air through the bottom of the depressions or grooves assists in the removal of the forming tool from the glass or plastic substrate.
According to a preferred embodiment of the method the viscous glass or plastic substrate is taken directly from a melt of the glass or plastic material.
According to an alternative preferred embodiment of the method a solid glass or plastic substrate is provided and prior to forming the microstructure the solid glass or plastic substrate is heated locally in order to plasticize the substrate in the region in which the microstructure is to be formed.
In the case of the second alternative of course additional energy for melting of the glass or plastic material is required when the substrate is present in solid form, however the simpler handling of the substrate material and independence from a melt are perceived as advantageous in certain applications.
In some embodiments of the forming tool the entire base body consists of a porous material with an open pore structure.
Alternatively for greater efficiency and technical operability, the forming tool can be formed so that only a portion of the base body adjacent or next to the operative layer consists of porous base material with an open pore structure, while the remaining portion is gas-impermeable.
According to another feature of the preferred embodiments of the invention the side walls of the porous base body are gas-impermeable. Because of that feature the full under-pressure or overpressure is provided in the depressions or grooves in the negative structure of the forming tool, i.e. only acts in the depressions or grooves, so that the forming of the microstructure in the substrate and the removal of the forming tool from it are optimized.
Advantageously the forming tool is a press roller or press tool according to the particular application.